A thermal dispersion probe typically includes two thermowell-protected RTD's (Resistance Temperature Detectors) which are placed into a medium (air, gas, liquids, slurries or solids) to be monitored. One RTD is preferentially heated while the other RTD senses the temperature of the medium, the temperature differential of the two RTD's is related to the medium flow rate as well as the properties of the medium. The principle of operation of the probe is based on the rate of dispersion of thermal energy from the heated RTD by the medium. As the flow-rate of the medium increases, more of the heat created by the heater by the heater is carried away resulting in a reduction of the temperature differential between the sensors. Using a well-known mathematical formula, the device uses the temperature differential between the RTD's to determine the flow rate of a particular medium or, given a constant flow rate, can determine the type of medium being measured. This data is then processed by devices such as a computer to effect control systems. The device may be utilized in virtually any condition as it may be paired with external software controls which can be downloaded into the device.
Current designs offer a single heater setting for the entire range of the RTD's, these designs cannot intelligently allocate the proper amount of thermal energy required in all necessary instances as it is either `full on` or `full off`. `Full on` results in wasted energy when the sensor is located in a medium of low specific gravity and additionally results in very slow response times to major changes in the medium movement or composition. Additionally, when physical jumpers are utilized to select heater power for specific sections of the flow spectrum, it unwittingly restricts the spectrum of the sensors' range. Any significant change in medium will require operator intervention.
In other words, current flow rate measurements may not be as accurate as necessary if the flow rate is either very high or very low. The heat source in the probe is designed to operate for all rates of flow. If the flow-rate is very high, most of the heat created by the heat source will be removed by the first flowing fluid before the thermistor has a chance to measure it. Therefore, small changes in the flow-rate at this end of the spectrum may not be noticed. Similarly, if the flow-rate is very low, most of the heat generated by the heat source will be measured by the thermistor. Too much heat has the same effect on the results as too little heat in that the smaller changes is flow may go unnoticed. Accordingly, where large fluctuations in flow rate are encountered, accurate measurements over the whole range is difficult.
Another shortcoming of present thermal dispersion switches is the lack of appropriate methods to test the switch to ensure it is operating properly. Even those switches that do provide a self-test still require some operator intervention. Therefore, a malfunction of the switch can still go undetected until the next scheduled operator test.
It is an object of the present invention to obviate or mitigate at least some of the above disadvantages.